Electrochemical sensor and method of forming thereof

ABSTRACT

Electrochemical sensors can include at least two electrodes, over which an electrolyte is formed. The electrodes can be isolated from one another in order for reduction/oxidation reactions to occur at the electrodes and for an electric current to flow therebetween. The present disclosure describes the use of a barrier in the electrochemical sensor that is configured to isolate electrodes from one another for the purpose of preventing electrode shorting. Additionally, the physical structure of the barrier can also act as a stencil for shaping the electrodes.

FIELD OF THE DISCLOSURE

The present disclosure relates to an electrochemical sensor having abarrier. The present disclosure also relates to a method of forming suchan electrochemical sensor having a barrier.

BACKGROUND

Electrochemical gas sensors can include a substrate upon which two ormore electrodes and an electrolyte reside. An example of such a sensoris disclosed in the applicant's co-pending application U.S. Ser. No.15/251,833 which is incorporated herein by reference. The electrodes orthe electrolyte are exposed to the natural environment by one or moreholes or pores provided in a portion of the housing. For example, aplurality of capillaries may be provided in a substrate upon which theelectrodes and electrolyte are formed. When certain gases enter thedevice via the openings, an electrochemical reaction occurs which may besensed by connections to the electrodes.

SUMMARY OF THE DISCLOSURE

Electrochemical sensors typically include at least two electrodes, overwhich an electrolyte is formed. The electrodes must be isolated from oneanother in order for reduction/oxidation reactions to occur at theelectrodes and for an electric current to flow therebetween. The presentdisclosure proposes the use of a barrier in the electrochemical sensor(hereinafter also referred to as the “device”) that is configured toisolate electrodes from one another for the purpose of preventingelectrode shorting. Additionally, the physical structure of the barriercan also act as a stencil for shaping the electrodes.

In accordance with a first aspect of the disclosure, there is providedan electrochemical sensor, comprising: a substrate having one or moregas transmission openings formed therein, the openings arranged to allowgases to pass through the substrate; two or more electrodes; a barrierfor the two or more electrodes; and an electrolyte formed over thebarrier and the two or more electrodes, wherein the barrier isconfigured to isolate at least part of one electrode from part ofanother electrode.

The barrier is a physical barrier that prevents at least part of anelectrode from flowing into other areas of the semiconductor device. Inparticular, the barrier acts to prevent at least part of an electrodefrom flowing into part of another electrode, in which case shortingwould occur between the two electrodes. The barrier also acts as anelectrical barrier between two electrodes; it can be made of a materialthat is electrically insulating, for example, a polymer such aspolyimide or a photoresist such as SU8. The barrier material ispreferably also photodefinable so that it can be easily printed onto asubstrate or upper layer of the device, or can be etched afterdepositing.

Aside from preventing the electrodes from shorting out, anotheradvantage that the barrier provides is allowing for the electrodes to beprinted closer together than in a device without the barriers, and thussmaller and cheaper sensors can be manufactured. The barriers also allowthe use of higher viscosity inks for printing the electrodes since theyisolate the electrodes from one another and so they prevent the flow ofelectrode ink from one electrode to another.

The two or more electrodes are formed in a pattern over an upper surfaceof the electrochemical sensor, wherein the upper surface may be aninsulating layer, a conductive track, an adhesive layer or a passivationlayer. One or more protrusions can also be formed over an upper layer ofthe electrochemical sensor and they can be formed at least between thetwo or more electrodes so as to be configured to act as barriers toprevent the two or more electrodes from contacting one another.

By “over”, it will be appreciated that this refers to the orientation ofthe electrochemical sensor as shown in the figures, rather than theorientation of the electrochemical sensor in use.

In order to isolate at least part of one electrode from part of anotherelectrode, a maximum height of the barrier may be at least equal to amaximum height of the two or more electrodes, and preferably the maximumheight of the barrier can be at least twice the maximum height of thetwo or more electrodes. In such a configuration, the height of thebarrier acts as a physical barrier to prevent parts of one electrodefrom flowing into part of another electrode.

The barrier height may range from 10 um to 200 um. The height requiredcan be determined by the thickness of the electrodes that needs to beisolated or separated and also the proximity of the electrodes whenprinted. In some examples, the barrier may be at least 25 μm in height.Alternatively, the barrier may be at least 50 μm in height.

For further robustness of the barrier, the barrier may be configured tobe well-shaped, having two peaks enclosing a dip, the two peakspreferably being spaced apart by at least 0.02 millimetres. In otherwords, the barrier may be double-walled. A lowest height of the dip maybe at least equal to the maximum height of the two or more electrodes.In the event that a part of one electrode overflows over an inner wallof the barrier, it may reside between the two walls of the barrier, i.e.in the dip, and is prevented from flowing over the outer wall of thebarrier.

The electrochemical sensor may further comprise an upper insulatinglayer having one or more openings configured to receive the two or moreelectrodes. The upper insulating layer may be, for example, apassivation layer. The barrier may be arranged over the insulatinglayer. In some alternative examples, the barrier may be formedintegrally with the passivation layer. The barrier may preferably beconfigured to also receive the two or more electrodes, such that the twoor more electrodes are at least partly defined by the barrier.

An outer perimeter of a barrier for an electrode may be built up as anadditional layer around an opening in the upper insulating layer andthen the electrode can formed within the barrier and within the openingof the upper insulating layer. Alternatively, the barrier may be formedas a “tub” within the opening in the upper insulating layer and anelectrode may be screen-printed or deposited into the tub. The walls ofthe tub prevent overflow of electrode into other areas of theelectrochemical sensor or another electrode. As an alternative tocreating the barrier in an additive process, the barrier could be formedby a removal process, for example, etching the upper insulating layersuch that the resulting patterned upper insulating layer forms thebarrier.

In photolithography or screen-printing techniques for printingelectrodes, a stencil is often required for outlining the shape of theelectrodes. However, in the above cases, the barrier may act as astencil for forming the two or more electrodes, and there is no need fora separate stencil.

The barrier may be arranged to surround at least one electrode.Preferably, the barrier may be arranged to surround and outline eachpart of the electrode. The barrier may also be arranged substantiallybetween at least two electrodes in order to isolate part of the two ormore electrodes from one another. Preferably, the barrier is arrangedbetween the electrodes so as to fully isolate each part of the twoelectrodes from one another.

The electrochemical sensor may further comprise a cap for housing theelectrolytes. The barrier may also be formed to align with protrusionson the cap such that, in use, the barriers also act to contain or housethe electrolyte. When the barrier is made of an adhesive or a bondingmaterial, they may also be used to attach the cap to an upper surface ofthe electrochemical sensor. Barriers may also be provided outside and/orsurrounding at least one electrode in order to protect the electrodefrom the cap.

In accordance with a second aspect of the disclosure there is providedan electrochemical sensor, comprising: a substrate having one or moregas transmission openings formed therein, the openings arranged to allowgases to pass through the substrate; two or more electrodes formed in apattern over an upper surface of the electrochemical sensor; one or moreprotrusions also formed over the upper surface of the electrochemicalsensor and formed at least between the two or more electrodes; anelectrolyte formed over the barrier and the two or more electrodes,wherein the one or more protrusions are configured to act as barriers toprevent the two or more electrodes from contacting one another.

As with the first aspect, the one or more protrusions act as physicalbarriers to prevent the two or more electrodes from contacting oneanother by preventing at least part of an electrode from flowing intopart of another electrode, which would result in shorting of theelectrodes. The barrier also acts as an electrical barrier between twoelectrodes.

The upper surface may be an insulating layer, a conductive track, anadhesive layer or a passivation layer. In one example, the barrier maybe built around an opening in the insulating layer that has been etchedaway.

In accordance with a third aspect of the disclosure there is provided amethod of forming an electrochemical sensor, the method comprising thesteps of: providing a substrate having one or more gas transmissionopenings, the openings arranged to allow gases to pass through thesubstrate; forming a barrier for two or more electrodes; forming two ormore electrodes; forming an electrolyte over the barrier and the two ormore electrodes, wherein the barrier is configured to isolate at leastpart of one electrode from part of another electrode.

Again, as with the first and second aspects, by forming barriers thatare configured to isolate at least part of one electrode from part ofanother electrode, shorting between the electrodes is avoided. Thebarrier also acts as an electrical barrier between two electrodes.

Typically, the barrier may be formed over an upper layer of theelectrochemical sensor by adding layers, or by removing or etching awaylayers. Some examples of these two processes are discussed as follows.

In order to process a barrier on an upper surface of the substrate byadding layers, a photo-definable polymer may be applied. This could bespun on or it could be laminated. The polymer is defined using aphotolithography process. In order to create the barriers in thesubstrate by removing layers, a pattern may be applied to the substratefirst to define “tubs”. This can also be done using a photolithographyprocess. The tubs in the substrate can then be etched away using a dryor wet etch. The mask for the etching process could be a resist polymeror it may be a hard mask such as an oxide. The tubs in the substrate maybe lined with an insulating layer such as an oxide in order to isolatethe electrode to be filled in the tubs from a conductive substrate suchas silicon.

In the method of providing a barrier between electrodes in anelectrochemical sensor, the method may further comprise one or more ofthe following steps:

-   -   forming the barrier using photolithography, lamination or        deposition in an additive process;    -   forming the barrier using etching in a removal process;    -   forming an upper insulating layer having one or more openings        configured to receive the two or more electrodes;    -   arranging the barrier over the upper insulating layer;    -   configuring the barrier to also receive the two or more        electrodes, such that two or more electrodes are at least partly        defined by the barrier;    -   forming a maximum height of the barrier to be at least equal to        a maximum height of the two or more electrodes, and preferably        forming the maximum height of the barrier to be at least twice        the maximum height of the two or more electrodes;    -   forming the barrier to be between 10-200 μm in height;    -   forming the barrier to be at least 25 μm in height;    -   forming a wall of the barrier to be well-shaped with two peaks        enclosing a dip, the two peaks preferably being spaced apart by        at least 0.02 mm;    -   forming a lowest height of the dip is at least equal to the        maximum height of the two or more electrodes;    -   arranging the barrier to surround at least one electrode; and    -   arranging the barrier substantially between at least two        electrodes.

In each of the above examples, advantages associated with one aspect ofthe disclosure may also be associated with another aspect of thedisclosure if appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present disclosure will now be described by non-limitingexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of screen-printed electrodes;

FIG. 2A is a cross-sectional view of an electrochemical sensor inaccordance with a first example of the disclosure;

FIG. 2B is a plan view of an electrochemical sensor in accordance withanother example of the disclosure;

FIG. 2C shows plan views of electrochemical sensors in accordance withother examples of the disclosure;

FIG. 3 is a cross-sectional view of an electrochemical sensor inaccordance with a second example of the disclosure;

FIG. 4 is a cross-sectional view of an electrochemical sensor inaccordance with a third example of the disclosure;

FIG. 5 is a cross-sectional view of an electrochemical sensor inaccordance with a fourth example of the disclosure;

FIG. 6 is a cross-sectional view of an electrochemical sensor inaccordance with a fifth example of the disclosure;

FIG. 7 is a cross-sectional view of an electrochemical sensor inaccordance with a sixth example of the disclosure;

FIG. 8A schematically illustrates a substrate at an initial phase of afabrication process for the electrochemical sensor shown in FIGS. 7A and7B;

FIG. 8B shows the substrate after formation of an insulating layer;

FIG. 8C shows the substrate after formation of microcapillaries;

FIG. 8D shows the substrate after formation of a metal layer;

FIG. 8E shows the substrate after deposition and definition of thepassivation layer;

FIG. 8F shows the substrate after removal of a portion of the insulatinglayer;

FIG. 8G shows the substrate after forming the barrier;

FIG. 8H shows the substrate after deposition of electrodes;

FIG. 8I shows the substrate after application of a cap;

FIG. 83 shows the substrate after insertion of an electrolyte andsealing of the cap,

FIG. 9 is a flow diagram showing the steps in a method in accordancewith another example of the disclosure;

FIG. 10 is a flow diagram showing options for additional steps in amethod in accordance with yet another example of the disclosure; and

FIG. 11 is a flow diagram showing options for additional steps in amethod in accordance with yet another example of the disclosure.

DETAILED DESCRIPTION

During manufacture, an electrochemical sensor may be filled with asuitable electrolyte. The electrolyte sits over the electrodes so as tofacilitate current flow between the electrodes. However, when theelectrode overspills into areas of the electrochemical sensor that isnot within its predefined region, or when one electrode overspills andcontacts another electrode, electrical shorting occurs, causing theoperation of the electrochemical sensor to potentially fail.

In the present disclosure, a part of one electrode is isolated from partof another electrode by a barrier, which prevents the two electrodesfrom contacting one another. Therefore, electrical shorting between theelectrodes is avoided and the electrochemical sensors utilising suchbarriers are much more stable over their working lifetime.

A barrier is an obstacle that keeps apart or prevents movement acrosstwo areas. In the context of the present electrochemical sensor, thebarrier is both a physical barrier and an electrical barrier between atleast two electrodes. Therefore, the barrier can be made of anelectrically insulating material, for example, a polymer such aspolyimide or a photoresist such as SU-8. The barrier material ispreferably also photodefinable so that it can be easily printed onto asubstrate or upper layer of the device, or can be etched afterdepositing.

The barrier may comprise one or more protrusions formed in a patternover an upper surface of the electrochemical sensor, the protrusionsreceiving the electrodes. The upper surface may be an insulating layer,a conductive track, an adhesive layer or a passivation layer. The one ormore protrusions can be formed at least between the two or moreelectrodes so as to be configured to act as barriers to prevent the twoor more electrodes from contacting one another.

The problem of electrical shorting is illustrated in FIG. 1, in whichelectrodes 10A and 10B are shown to contact one another. When inks fromthe two electrodes flow into one another, beyond their own predefinedareas, electrical shorting occurs and detection of oxidation andreduction of gas within the electrochemical sensor breaks down. Incontrast, electrodes 20 a and 20 b are printed neatly within theirpredefined areas due to the presence of a barrier 20 c and so theoxidation and reduction of gas within the electrochemical sensor invokescurrent flow between electrodes 20 a and 20 b, thus allowing efficientoperation of the electrochemical sensor.

The electrochemical sensor may have two or more electrodes. Typically,at least two electrodes are provided; a working electrode and a counterelectrode. The potential difference, current flow or resistance betweenthese electrodes may be measured in order to determine whether a gas hasentered through openings in the substrate of the device. Sometimes, athird electrode, known as a reference electrode, is also provided. Thereference electrode is held at a constant potential with respect to theworking electrode. The presence of substances which interact with theworking electrode/electrolyte interface can invoke current flow betweenthe working electrode and the counter electrode as a result ofreduction/oxidation reactions at the working electrode. Additionalelectrodes such as a diagnostics electrode and/or a second workingelectrode, etc. may also be incorporated.

FIG. 2A shows a cross-section through an electrochemical sensor 100formed on silicon using micromachining techniques in accordance with afirst example of the disclosure. An example of such a sensor isdisclosed in the applicant's co-pending application U.S. Ser. No.15/251,833 which is incorporated herein by reference. Theelectrochemical sensor is formed on a silicon substrate 101. In thisexample, a single sensor is formed on the silicon substrate 101.However, in practice, several sensors may be formed on a singlesubstrate, in a similar manner to the way in which multiple integratedcircuits may be formed on a single silicon substrate. As an alternativeto silicon, the substrate may be made from glass, ceramic or plastic. Aplurality of microcapillaries 102 are formed in the substrate 101. InFIG. 2A, six microcapillaries are shown in cross-section. However, themicrocapillaries 102 are also formed across the width of the substrate,and there may be typically ten or more microcapillaries, or a singlemicrocapillary. Each microcapillary is formed in a direction orthogonalto the surface of the substrate 101, and extends from an upper surfaceto a lower surface of the substrate. Each microcapillary isapproximately 20 μm in diameter, although each microcapillary may be inthe range of 1 μm to 2 mm in diameter. The group of microcapillaries 102is approximately 1 mm across, but may be in the range of 0.001 mm to 3mm across.

An insulating layer 103 is formed on the upper surface of the substrate101. The insulating layer 103 may be formed from silicon oxide (SiO2)and is approximately 4 μm thick. An electrode opening 104 is formed inthe insulating layer 103 in a position that is aligned with themicrocapillaries 102. The opening is described as being aligned in thesense that the microcapillaries are formed in an area defined by theopening in the insulating layer. The walls of the opening 104 are notnecessarily precisely aligned with the walls of the microcapillaries. Inthis example, the opening 104 is approximately circular, but may besquare or rectangular. The opening 104 may be 1 to 2 mm across. The sidewalls of the opening 104 are straight in shape. However, it will beappreciated that the side walls may be semi-circular or may be formedfrom any other shape that increases the surface area of the side walls.

Conductive tracks 105A, 105B are formed on a top surface of theinsulating layer 103. The conductive tracks 105A, 105B are adhered tothe insulating layer 103 by an adhesion layer 106A, 106B. The conductivetracks 105A, 105B may be made of gold or any other suitable conductivematerial. For example, the conductive tracks may be made from metal orconductive plastic. The conductive tracks are arranged such that theystop approximately 25 μm from the edge of the opening 104. The tracksmay stop anywhere between a few microns to a few millimeters from theedge of the opening. The conductive tracks 105A, 105B are for connectingthe electrodes to external circuit elements. The conductive tracks mayextend into the opening formed in the insulating layer 103. Additionallythe conductive tracks may extend into the capillaries in order toimprove contact resistance.

A passivation layer 107 is formed over the insulating layer 103 and theconductive tracks 105A, 105B. An opening 108 is formed in thepassivation layer 107. The opening 108 is the same size as the electrodeopening 104, and is aligned with the opening 104. Additional holes 109A,109B, 109C, 109D are formed in the passivation layer to allowconnections to be made between the electrodes (discussed below) andexternal circuit elements. Additional holes may be added for sensorswith more than two electrodes.

As FIG. 2A shows a cross-section through the sensor 100, only a workingelectrode 110A and a counter electrode 110E are shown. The workingelectrode 110A is formed in the openings 104 and 108. The electrodecompletely fills the openings 104 and 108 and abuts the top surface ofthe substrate 101.

The working electrode 110A extends approximately 25 μm above the top ofthe passivation layer 107. The working electrode 110A also extends intohole 109B. This provides an electrical connection to conductive track105B, allowing connections to external circuit elements via hole 109A. Acounter electrode 110B is formed in hole 109C. Counter electrode 110Ealso extends 25 μm above the passivation layer 107. The counterelectrode 110B also extends into hole 109C. This provides an electricalconnection to conductive track 105A, allowing connections to externalcircuit elements via hole 109D. The electrode 110A is printed directlyon the microcapillaries 102. As such, the electrolyte 114 may be liquid.The electrode 110A prevents the electrolyte 114 passing through themicrocapillaries. The electrodes are porous and are made of a catalyst,such as platinum. The electrode 110A thus provides the 3-phase poroussurface required for the chemical reactions to take place. The catalystis a medium to high surface area porous catalyst, such as platinumblack. Sufficient catalyst is provided to ensure sufficient catalyticactivity throughout the sensor's lifetime. The catalyst may also be oneof platinum, gold, ruthenium, carbon black or iridium. Other appropriatematerials may be used.

A barrier 120 is provided for the electrodes 110A, 110B, wherein thebarrier 120 is configured to isolate the electrodes 110A, 1108 from oneanother. The barrier 120 is a physical barrier that prevents at leastpart of an electrode from flowing into other areas of the semiconductordevice. In particular, the barrier forms a pattern of protrusions abovethe passivation layer 107 and acts to prevent at least part of anelectrode from flowing into part of another electrode and contacting theother electrode, in which case shorting would occur between the twoelectrodes. The barrier 120 also acts as an electrical barrier betweentwo electrodes; it can be made of a material that is electricallyinsulating. In the example of FIG. 2A, the barrier 120 is made ofpolyimide, which is photodefinable so that it can be easily printed ontoa substrate or upper layer of the device 100, or can be etched afterdepositing.

Aside from preventing the electrodes 110A, 1108 from shorting out,another advantage that the barrier 120 provides is allowing for theelectrodes to be printed closer together than in a device without thebarriers, and thus smaller and cheaper sensors can be manufactured. Thebarrier 120 also allows the use of higher viscosity inks for printingthe electrodes since they isolate the electrodes 110A, 110E from oneanother and so they prevent the flow of electrode ink from one electrodeto another.

In FIG. 2A, in order to isolate the electrodes 110A, 1108 from oneanother, a maximum height of the barrier 120 is equal to a maximumheight of the electrodes 110A, 110B. In such a configuration, the heightof the barrier 120 acts as a physical barrier to prevent parts of oneelectrode from flowing into part of another electrode. The barrierheight may range from 10 μm to 200 μm, but in the specific example ofFIG. 2A, the barrier height is 25 μm in height.

The barrier 120 is built up as an additional layer around an opening inthe passivation layer 107 and then the electrodes 110A, 110B are formedwithin the barrier 120 and within the opening of the passivation layer107. In standard photolithography techniques for printing electrodes110A, 110B, a stencil is often required for outlining the shape of theelectrodes 110A, 110B. However, in FIG. 2A, the barrier 120 acts as astencil for forming the two electrodes 110A, 110B, and there is no needfor a separate stencil.

A cap 111 is formed over the electrodes 110A, 110B. In embodiments whereadditional electrodes are used, the cap 111 would also be formed overthose electrodes. The cap may be formed from glass, ceramic, silicon orplastic. The cap 111 is sealed to the passivation layer 107 byepoxy/adhesive or frit glass 112A, 112B. Other bonding techniques may beused. A hole 113 is formed in the top of the cap 111. An electrolyte 114is provided within the cap 111. In another aspect, two or more holes maybe formed in the cap 111. This would enable the electrolyte to be vacuumfilled. Alternatively, the electrolyte can be dispensed using a jetterdispensing nozzle through a fill hole and the air inside the cavity canbe displaced through a vent hole. The electrolyte 114 may be made from aliquid solution, such as a conductive aqueous electrolyte or organicelectrolyte, a conductive polymer, such as Nafion or PEDOT:PSS. Theelectrolyte may also be a hydrogel or a room temperature ionic liquid.In one example, the electrolyte may be sulfuric acid solution and mayinclude a wicking material or wicking substructure. The electrolyte maybe a two-layer electrolyte. The electrolyte 114 completely covers theelectrodes, but when using liquid electrolytes, does not completely fillthe cap 112. Instead, a void space 115 is left towards the top of thecap 111. The void space 115 may not be required when using conductivepolymer electrolytes, hydrogels and some other non-aqueous electrolytes.Epoxy glue or a sealing tape 116 (or any other organic polymericmaterial) is formed over the hole or holes 113 to prevent or restrictany pollutants entering the cap, and also to prevent or restrict theelectrolyte 114 from leaving the cap. Other options may be utilized forsealing. If two holes are provided in the cap 111, a seal may be formedover both holes. In another aspect, a larger hole could be covered withan adhered lid, once the cavity is filled.

If the cap 111 is made from plastic, the plastic material must becompatible with the electrolyte 114. Various plastic materials may beused. For example, the cap may be made from acrylonitrile butadienestyrene (ABS), PTFE, polycarbonate (PC), polyethylene (PE),polydimethylsiloxane (PDMS), amongst other plastics. Importantproperties of the plastic are its chemical resistance and itscompatibility with the electrolytes.

In FIG. 2A, the conductive tracks 105A, 105B are provided over theinsulating layer 103. The openings 109A, 109D are provided outside ofthe cap 111 in order to allow the sensor to be connected to externaldevices. It may be preferable to omit the portion of the substrate 101and insulting layer 103 that extend outside of the cap 111, in order toreduce the size of the sensor 100. In order to facilitate this, theconductive tracks may be omitted, and conductive vias may be formedthrough the substrate instead. This would enable connections to be madeon the underside of the substrate 101. Additionally, the size of thesubstrate 101 may be reduced to the size of the cap 111.

The microcapillaries 102 may be lined with an insulating material. Thepurpose of this would be to electrically insulate the silicon substrate101 from the electrodes.

FIG. 2B shows a plan view of an example sensor 200 with the cap 111 andthe electrolyte 114 removed for clarity. The configuration of the sensorconductive tracks and electrodes in FIG. 1B slightly differs from thatshown in FIG. 2A their shape and arrangement relative to the othersensor components. In FIG. 2B, the sensor 100 also includes conductivetracks 206A, 206B and 206C, The conductive tracks are shown in brokenlines, as they are all positioned below the passivation layer.Conductive track 206A is for connecting the working electrode 204A, Theconductive track includes a ring-shaped portion, which is located aroundthe capillaries 202, but within the outer edge of the working electrode204A, The ring-shaped portion is co-axial with the working electrode204A. A ring-shaped opening is formed in the passivation layer, and isaligned with the ring-shaped portion of the conductive track 206A, inorder to allow the working electrode 204A to connect to the conductivetrack 206A. A rectangular connecting portion of track 206A is formed atthe bottom edge of the ring-shaped portion, to provide a connection toexternal circuitry.

Conductive tracks 206B and 206C are formed partially underneath counterelectrode 204B and reference electrode 204C respectively. Each trackincludes a semi-annular portion which is the same shape as thecorresponding electrode, but slight smaller in size. As such, thesemi-annular portions fit within the perimeters of their respectiveelectrodes. Openings are provided in the passivation layer to enable theconductive tracks 206B and 206C to connect to the working electrode 204Band reference electrode, respectively. These openings are similar insize and shape to the semi-annular portions of the conductive tracks205E and 206C. In a similar manner to the conductive track 206A, theconductive tracks 206B and 206C include rectangular portions whichextend from an outer edge of the semi-annular portions to provideconnections to external circuitry.

The purpose of using a circular and semi-annular arrangement is toreduce and optimise the distance and spacing between the electrodes.This reduces the resistance path between the electrodes, which canaffect the sensor performance, including speed of response. For example,in a carbon monoxide sensor, there's ion movement, or transport, betweenthe electrodes in the sensor. Ideally, therefore, the electrodes(including the entire electrode area) should be as close together aspossible. Using circular and semi-annular electrodes makes this easierachieve.

As can be seen from FIG. 2B, the barrier 120 is arranged to surround atleast one electrode. By providing the barrier 120 outside andsurrounding the electrode 1108, the barrier 120 also protects theelectrode 110B from the cap of the electrochemical sensor.

The barrier 120 is also shown to be arranged substantially between theelectrodes 110A, 110B in order to isolate them from one another. Thebarrier 120 also allows the use of higher viscosity inks for printingthe electrodes since it fully isolates the electrodes 110A, 1108 fromone another and so they prevent the flow of electrode ink from oneelectrode to another. As a result, another advantage that the barrier120 provides is allowing for the electrodes 110A, 110B to be printedcloser together than in a device without the barriers, and thus smallerand cheaper sensors can be manufactured.

FIG. 2B shows a sensor with components that have particular relativedimensions. These dimensions may be altered. The length and width ofeach sensor may be in the range of 1 mm to 10 mm. The overall thickness,including the substrate 101 and the cap 111 may be 1 mm. As such, on atypical 200 mm wafer, in excess of 1000 sensors may be produced.

In use, the sensor would be connected to a micro-controlled measurementsystem in a manner familiar to those skilled in the art. The sensoroutput may be continuously monitored and used to determine theconcentration of analyte in the environment. The electrode 110A may comeinto contact with environmental gases via the microcapillaries 102. Asthe electrode 110A is porous, the environmental gases are able to passthrough the electrode to a point where they come into contact with theelectrolyte 114. A three-phase junction is therefore formed within theelectrode. An advantage of using a printed, solid electrode 110A, isthat it prevents or restricts the electrolyte 114 from escaping throughthe microcapillaries 102 in the substrate 101.

An advantage of the above-described structure is that siliconmicromachining techniques can be used in its construction. As such,manufacturing of the sensor is compatible with fabrication techniquesused to manufacture integrated circuits. By manufacturing multiplesensors in parallel, variations in the parameters of the sensors arereduced.

A further advantage of using silicon fabrication techniques is that thecost of each device is reduced. This is because each process step isapplied to multiple sensors in parallel, so the processing cost perdevice is small. Additionally, micromachining techniques enable verysmall devices to be produced. As such, the sensors may be more easilyincorporated into handheld devices. Furthermore, the sensors all see thesame processing steps at the same time. As such, matching betweendevices is very good when compared with serially produced devices.

FIG. 2C shows some further examples of plan views of electrodes ofelectrochemical sensors in accordance with other examples of thedisclosure. In these examples, the barriers are shown in differentarrangements to FIG. 2B. However, they still all act to isolate at leastpart of one electrode from part of another electrode, whether that is bysurrounding an electrode and/or by being arranged substantially betweentwo electrodes.

FIGS. 3 to 7 show alternative examples of the disclosure to FIG. 2A andlike components with FIG. 2A are labelled with like reference numerals.Technical effects and advantages associated with like components of FIG.2A may therefore also apply to FIGS. 3 to 7.

FIG. 3 shows a cross-section through an electrochemical sensor 100formed on silicon using micromachining techniques in accordance with asecond example of the disclosure.

In FIG. 3, the difference compared with FIG. 2 is that the barrier 121extends beyond the height of the electrodes 110A, 110B. The maximumheight of the barrier 121 is about twice the maximum height of theelectrodes 110A, 110B when measured from the plane of the passivationlayer 107 and the barrier 121 is 50 μm in height. By using a barrier 121that has a maximum height that is greater than the maximum height of theelectrodes 110A, 110B, the effectiveness of the barrier 121 as a meansto isolate the electrodes 110A, 1108 from one another is increased.

FIG. 4 shows a cross-section through an electrochemical sensor 100formed on silicon using micromachining techniques in accordance with asecond example of the disclosure.

The barrier 122 of FIG. 4 is formed to align with protrusions on the cap111 of the electrochemical sensor 100 such that, in use, the barrier 122also acts to contain or house the electrolyte 114. An advantage of thisarrangement is that the barrier also confines the electrolyte over thecatalyst. This could reduce or eliminate the need for a wicking materialfor keeping the catalyst in contact with the electrolyte.

FIG. 5 shows a cross-section through an electrochemical sensor 100formed on silicon using micromachining techniques in accordance with asecond example of the disclosure.

When the barrier 123 is made of an adhesive or a bonding material, as isthe case in FIG. 5, it is also used to attach the cap 111 to the uppersurface of the passivation layer 107. Therefore, there is no need forthe epoxy/adhesive or frit glass 112A, 112B of FIG. 2A. By doing this,the number of processing steps are reduced, i.e. by removing theseparate adhesive step, which has the benefit of lowering the processingcost.

FIG. 6 shows a cross-section through an electrochemical sensor 100formed on silicon using micromachining techniques in accordance with asecond example of the disclosure.

For further robustness, in FIG. 6, the barrier 124 are configured to bewell-shaped, having two peaks enclosing a dip. The two peaks arepreferably spaced apart by at least 0.02 millimetres. The barrier 124 iseffectively double-walled. Therefore, in the event that a part of oneelectrode overflows over an inner wall of the barrier 124, it may residebetween the two walls of the barrier 124, i.e. in the dip, and isprevented from flowing over the outer wall of the barrier 124.

FIG. 7 shows a cross-section through an electrochemical sensor 100formed on silicon using micromachining techniques in accordance with asecond example of the disclosure.

The barrier 125 of FIG. 7 combines the advantages of the barrier 121 ofFIG. 3 and the barrier 124 FIG. 6, i.e. it has a maximum height ofdouble the maximum height of the electrodes 110A, 110E when measuringfrom the plane of the passivation layer 107, and it also isdouble-walled. Therefore, in order for contact between the electrodes110A, 110B to occur, the first hurdle that the electrode ink mustovercome is to spill over a barrier 125 that has a maximum height oftwice the maximum height of the electrodes 110A, 110E when measured fromthe plane of the passivation layer 107. By using a barrier 125 that hasa maximum height that is greater than the maximum height of theelectrodes 110A, 110B, the effectiveness of the barrier 125 as a meansto isolate the electrodes 110A, 1108 from one another is alreadyincreased. Further, the second hurdle is, even if a part of oneelectrode overflows over an inner wall of the barrier 125, it may residebetween the two walls of the barrier 125, i.e. in the dip, and isprevented from flowing over the outer wall of the barrier 125. A lowestheight of the dip, as shown in FIG. 6, is equal to the maximum height ofthe two electrodes 110A, 110B, for effective containment of anyelectrode ink that overspills into the dip. Together, the aggregateeffect of the barrier 125 is a particularly robust and reliable means toisolate the electrodes 110A, 110E of the electrochemical sensor 100.

A method of fabricating the electrochemical sensor 100 will now bedescribed with reference to FIGS. 8A to 8J.

FIG. 8A shows the first step in the fabrication process. A silicon waferis used as the silicon substrate 101. In the following, the process forforming one device will be described, however several hundred devicesmay be formed in parallel on the same wafer. The silicon substrate 101is used for mechanical support, and could be substituted for anothertype of material, such as glass.

An oxide insulating layer 103 is deposited on the wafer, as shown inFIG. 8B. The oxide layer serves as a “landing” oxide to stop the throughwafer etch, and also serves as a layer to insulate the conductive tracksfrom the substrate to prevent shorting.

The microcapillaries 102 are defined in the wafer by photolithography.The microcapillaries are etched through the wafer using an isotropic dryetch. They are etched from the backside of the wafer and stop at theoxide layer once the silicon wafer has been etched through, as shown inFIG. 8C.

FIG. 8D shows formation of inert metal layers which form the conductingtracks 105. They are deposited on the insulation layer, on the frontside of the wafer.

An adhesive layer 106 is first deposited on the insulating layer 103,and is used to attach the metal layer to the insulating layer 103. Theconductive tracks may be defined by photolithography and then etched.The thickness of the inert metal can be increased by electroplating inspecific areas, as defined by photolithography.

FIG. 8E shows the sensor after deposition and definition of thepassivation layer 107. The insulating oxide 103 on the front side of thewafer 101 is removed in the region of the microcapillaries 102 using awet etch, as shown in FIG. 8F.

In FIG. 8G, a barrier 125 is deposited on the upper surface of thepassivation layer in a pattern by photolithography in an additiveprocess. The barrier 125 is a double-walled barrier, wherein the dipbetween the two peaks occurs naturally when baking a polymer barrier andcan be further etched in order to optimise the shape of the dip. Barrier125 creates a stencil for the deposition of electrodes in the next step.

Although not shown in FIG. 8G, an alternative to creating the barrier inan additive process, the barrier could be formed by a removal process,for example, etching the passivation layer such that the resultingpatterned passivation layer forms the barrier.

A porous electrode material is deposited on the wafer using screenprinting, stencil printing, electroplating, or other lithographicdeposition techniques to form electrodes 110A and 110B as shown in FIG.8H. Electrode 110A covers the microcapillaries 102, and connection ismade to the conductive tracks. Electrodes 110A and 110B both sit withinthe barrier 125. Therefore, the electrodes 110A and 110B are isolatedfrom one another and prevented from contacting one another.

The cap 111 is then placed over the sensor 100, as shown in FIG. 8I. Asdescribed above, the cap 111 may be made of plastic, ceramic, silicon orglass, amongst other materials. If the cap is made of plastic, it istypically prefabricated by injection molding, or mold casting if usingPDMS, etc. The cap is coated on its inner surface with a hydrophobiclayers 203, 204, which repel any electrolyte that is filled in thecavity. The recess and holes may be formed during the injection moldingprocess. If the cap is made from glass, silicon or ceramic, the capwould typically be fabricated using wafer level processing techniques.For glass or ceramic caps, cavities can be made in the cap by firstlyusing photolithography to pattern the cap cavity. Then one of, or acombination of, wet etching, dry etching, sand blasting and laserdrilling may be used to create the cavities in the cap. For siliconcaps, cavities can be made in the cap by firstly using photolithographyto pattern the cap cavity. Then one of, or a combination of, wetetching, dry etching, sand blasting, and laser drilling may be used tocreate the cavities in the cap.

The cap 111 is attached to the wafer through wafer bonding (waferprocessing) or through placement with epoxy/adhesive on the sensor wafer(single cap placement process). Alternatively, the cap 111 may beattached by other means such as ultrasonics. The electrolyte 114 isdispensed through the cap hole 113 and the hole is sealed, as shown inFIG. 8J. As noted above, the cap 111 may have more than one hole.

FIG. 9 is a flow diagram illustrating various steps in a method ofmanufacturing an electrochemical sensor according to an example of thedisclosure. The method initially involves, at step S101, providing asubstrate having one or more gas transmission openings, the openingsarranged to allow gases to pass through the substrate. Then, at stepS102, two or more electrodes are formed. Then, at step S103, a barrieris formed for the two or more electrodes, wherein the barrier isconfigured to isolate at least part of one electrode from part ofanother electrode. Finally, at step S104, an electrolyte is formed overthe barrier and the two or more electrodes.

FIG. 10 is a flow diagram illustrating various options for additionalsteps in the method, in particular, for forming the barrier. At stepS105, a maximum height of the barrier is formed to be at least equal toa maximum height of the two or more electrodes, wherein preferably saidheight is at least 25 μm. At step S106, in order to create adouble-walled barrier, a wall of the barrier is formed to be well-shapeswith two peaks enclosing a dip, the two peaks preferably being spacedapart by at least 0.02 millimetres. At step S107, a lowest height isformed to be at least equal to the maximum height of the two or moreelectrodes. Steps S105, S106 and S107 result in an electrochemicalsensor that is particularly robust and effective in isolating two ormore electrodes from one another and preventing contact between twoelectrodes.

FIG. 11 is a flow diagram illustrating further options for additionalsteps in the method, in particular, for arranging the barrier relativeto the electrodes. In step S108, the barrier is arranged to surround atleast one electrode. Sometimes, the barrier may be arranged to surroundand outline each part of the electrode. At step 109, the barrier isarranged to be substantially between at least two electrodes. Sometimes,the barrier is arranged between the electrodes so as to fully isolateeach part of the two electrodes from one another.

In each of the above-mentioned examples of the disclosure, theelectrochemical sensors are shown in a horizontal orientation. It willbe appreciated that, in use, the electrochemical sensors may be arrangedin a non-horizontal orientation, for example, in a vertical orientationor at an angle to the horizontal orientation shown in the Figures.

The above description relates to particularly preferred aspects of thedisclosure, but it will be appreciated that other implementations arepossible. Variations and modifications will be apparent to the skilledperson, such as equivalent and other features which are already knownand which may be used instead of, or in addition to, features describedherein. Features that are described in the context of separate aspectsor examples may be provided in combination in a single aspect orexample. Conversely, features which are described in the context of asingle aspect or example may also be provided separately or in anysuitable sub-combination.

The present disclosure had been described in the context of anelectrochemical sensor. However, it will be appreciated that theprincipals of using a barrier may also be applied to othermicroelectromechanical systems. Furthermore, microfluidic devices mayalso benefit from use of the barrier of the present disclosure.

1. An electrochemical sensor, comprising: a substrate having one or moregas transmission openings formed therein, the openings arranged to allowgases to pass through the substrate; two or more electrodes; a barrierfor the two or more electrodes; and an electrolyte formed over thebarrier and the two or more electrodes, wherein the barrier isconfigured to isolate at least part of one electrode from part ofanother electrode.
 2. An electrochemical sensor according to claim 1,wherein a maximum height of the barrier is at least equal to a maximumheight of the two or more electrodes, and preferably wherein the maximumheight of the barrier is at least twice the maximum height of the two ormore electrodes.
 3. An electrochemical sensor according to claim 1,wherein the barrier is at least 25 μm in height.
 4. An electrochemicalsensor according to claim 1, wherein a wall of the barrier iswell-shaped and has two peaks enclosing a dip.
 5. An electrochemicalsensor according to claim 4, wherein a lowest height of the dip is atleast equal to the maximum height of the two or more electrodes.
 6. Anelectrochemical sensor according to claim 1, wherein the electrochemicalsensor further comprises an upper insulating layer having one or moreopenings configured to receive the two or more electrodes.
 7. Anelectrochemical sensor according to claim 6, wherein the barrier isarranged over the upper insulating layer.
 8. An electrochemical sensoraccording to claim 7, wherein the upper insulating layer is apassivation layer.
 9. An electrochemical sensor according to claim 7,wherein the barrier is preferably configured to also receive the two ormore electrodes, such that two or more electrodes are at least partlydefined by the barrier.
 10. An electrochemical sensor according to claim1, wherein the barrier acts as a stencil for forming the two or moreelectrodes.
 11. An electrochemical sensor according to claim 1, whereinthe barrier is arranged to surround at least one electrode.
 12. Anelectrochemical sensor according to any claim 1, wherein the barrier isarranged substantially between at least two electrodes.
 13. Anelectrochemical sensor according to claim 1, wherein the electrochemicalsensor further comprises a passivation layer and the barrier is formedover the passivation layer.
 14. An electrochemical sensor, comprising: asubstrate having one or more gas transmission openings formed therein,the openings arranged to allow gases to pass through the substrate; twoor more electrodes formed in a pattern over an upper surface of theelectrochemical sensor; one or more protrusions also formed over theupper surface of the electrochemical sensor and formed at least betweenthe two or more electrodes; an electrolyte formed over the barrier andthe two or more electrodes, wherein the one or more protrusions areconfigured to act as barriers to prevent the two or more electrodes fromcontacting one another.
 15. A method of forming an electrochemicalsensor, the method comprising the steps of: providing a substrate havingone or more gas transmission openings, the openings arranged to allowgases to pass through the substrate; forming a barrier for two or moreelectrodes; forming the two or more electrodes; forming an electrolyteover the barrier and the two or more electrodes, wherein the barrier isconfigured to isolate at least part of one electrode from part ofanother electrode.
 16. A method according to claim 15, wherein formingthe barrier comprises forming the barrier using photolithography,lamination or deposition in an additive process.
 17. A method accordingto claim 15, wherein forming the barrier comprises forming the barrierusing etching in a removal process.
 18. A method according to claim 15,wherein the method further comprises forming an upper insulating layerhaving one or more openings configured to receive the two or moreelectrodes.
 19. A method according to claim 18, wherein forming thebarrier comprises arranging the barrier over the upper insulating layer.20. A method according to claim 19, wherein forming the barriercomprises configuring the barrier to also receive the two or moreelectrodes, such that two or more electrodes are at least partly definedby the barrier.